Growth of Brandenburg-Prussia, 1600–1795
When Albert Frederick, Duke of Prussia, died without a son in 1618, his son-in-law John Sigismund inherited the Duchy of Prussia. He then ruled both territories in a personal union which came to be known as Brandenburg-Prussia. In this way, the fortuitous marriage of John Sigismund to Anna of Prussia, and the deaths of her maternal uncle in 1609 and her father in 1618 without immediate male heirs, proved to be the key events by which Brandenburg acquired territory both in the Rhineland and on the Baltic coast.
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